The primary objective of this study is to demonstrate the clinical efficacy of ORTHOCLONE OKTcdr4a in subjects with moderate to severe psoriasis. An additional objective is to demonstrate the safety and tolerability of ORTHOCLONE OKTcdr4a in this subject population. This is a multiple intravenous infusion, randomized, double-blind, placebo- controlled, multicenter study to demonstrate the clinical efficacy of ORTHOCLONE OKTcdr4a in subjects with moderate to severe psoriasis. Thirty-six subjects willbe randomized into three groups of 12 subjects each. There will be a maximum of two courses of study medication administration. Each group will receive a single course of three fixed dose infusions (total volume 50 mL) of study medication administered on Days 1,3, and 5, followed by a six month follow-up period. Group 1 will be randomized to receive placebo for the first course of study medication administration, Group 2 will be randomized to receive the low dose (225 mg/course) regimen of ORTHOCLONE OKTcdr4a, and Group 3 will be randomized to receive the high dose (750 mg/course) regimen of ORTHOCLONE OKTcdr4a. One retreatment course of high dose ORTHOCLONE OKTcdr4a (750 mg/course) will be allowed for any subject who does not have a >=50% decrease in their PASI score one month post-treatment. Subjects will be assessed for a period of six months after the last injection.